The OEM uses a type of sealant that is available over the counter as "Right Stuff", IIRC.

The gasket shouldn't require any sealant on the seals, just in the corners where the stamping may be a bit off. If the gasket was used w/a different pan than what you are NOW using, the seals may have taken a different 'set' that will allow the new pan to leak.

Also, if the gasket was ever overtightened, it could have been squeezed out of shape enough to cause problems.